Cruise control modulator

ABSTRACT

A modulated vehicle cruise control system includes a source of vacuum, a servo transducer or control unit connected to the source of vacuum, an engine throttle position servo connected to the servo, a speed sensor and a logic circuit or command module. A modulator is disposed in series with and between the control unit and the source of vacuum. The modulator limits opening movement of the engine throttle by modulating communication of vacuum to the servo to prevent inefficient engine operation.

This application is a divisional of prior co-pending application Ser.No. 204,066, filed Nov. 4, 1980, now U.S. Pat. No. 4,407,385.

BACKGROUND OF THE INVENTION

The present invention relates to vehicle cruise control systems and moreparticularly to a unique device for modulating constant speed cruisecontrol systems.

A wide variety of cruise or speed control systems have been proposed formaintaining the speed of a vehicle fairly constant. Such systemsautomatically vary the throttle position of the carburetor in responseto changing load conditions to maintain a constant set speed. Typicalcommercially available systems include a speed sensor which sensesactual vehicle speed and generates a signal which is transmitted to anelectronic logic circuit or module. The module compares actual vehiclespeed with a desired or "set" speed and generates a control signal whichis transmitted to a vacuum servo transducer or control unit. Thetransducer is typically a valve structure interposed between a vacuumstorage tank or a line connected to the engine intake manifold and avacuum actuator. The vacuum actuator is connected to the vehiclethrottle linkage. If the sensed speed is below the set speed, the servotransducer will "open" and vacuum from the storage tank will be appliedto the vacuum actuator causing the throttle to open until the "set"speed is reached and/or to maintain the set speed.

When a rapid increase in load occurs, such as when the vehicleapproaches a steep incline, typical cruise control systems produce asomewhat violent throttle position change. This violent change canactuate the accelerator pump or enrichment circuits of the carburetorresulting in excessive fuel flow to the intake manifold. This causesinefficient engine operation and a fairly rapid decrease in the intakemanifold vacuum level.

Under constant or steady state load conditions, vehicle cruise controldevices tend to increase fuel efficiency. Operator induced speed changeswhich have an adverse effect on fuel efficiency are avoided. However,under increasing load conditions, besides the occurrence of violentthrottle position changes, the cruise control systems will reduce intakemanifold vacuum levels, through throttle opening, in an attempt tomaintain vehicle speed. This also results in a decrease in engineoperating efficiency. Examples of automatic vehicle speed or cruisecontrol systems of the aforementioned type may be found in U.S. Pat. No.3,485,316, entitled AUTOMOBILE SPEED CONTROL and issued on Dec. 23,1969, to Slavin et al and U.S. Pat. No. 4,170,274, entitled DEVICE FORREGULATING THE TRAVELING SPEED OF A MOTOR VEHICLE and issued on Oct. 9,1979, to Collonia.

The intake manifold vacuum level of a spark ignition internal combustionengine is proportional to and a direct indicator of operating or fuelefficiency. An internal combustion engine operates most efficientlyunder conditions of high intake manifold vacuum. During rapidacceleration and/or high load conditions, intake manifold vacuum levelswill drop causing inefficient operation.

Various proposals have been made to indicate to the operator when thevehicle is being operated in an efficient range of intake manifoldvacuum levels. One such device is merely a vacuum gauge which delineatesefficient and inefficient "areas" of operation. The gauge would bemounted in the vehicle compartment and directly indicates intakemanifold vacuum levels. The operator attempts to "modulate" or controldepression of the accelerator to maintain intake manifold vacuum levelsat the "high" range to thereby increase fuel efficiency.

Intake manifold vacuum level sensors have also been electricallyconnected to a light signaling device which turns "on" duringinefficient vehicle operation. Such devices, of course, have no effecton the operation of an automatic cruise control system which may beincorporated in the vehicle.

Other devices have been proposed which actively indicate inefficientvehicle operation in response to a reduction in intake manifold vacuumlevels. These devices sense the intake manifold vacuum level andgenerate a force which tends to close the throttle valve and create anincreased resistance to accelerator movement. An example of one suchdevice may be found in U.S. Pat. No. 4,026,255, entitled VEHICLE ENGINESIGNAL DEVICE and issued on May 31, 1977, to Weiler, Jr. The devicedisclosed therein includes a piston cylinder type actuator which isconnected to the throttle linkage and is shifted in response to thechange in intake manifold vacuum levels. When a "low" vacuum level isencountered, the actuator tends to close the throttle causing increasedresistance to depression of the accelerator and thereby activelysignaling inefficient vehicle operation. Examples of other suchsignaling devices may be found in U.S. Pat. No. 2,148,729, entitledCARBURETOR THROTTLE CONTROL and issued on Feb. 28, 1939, to Coffey; U.S.Pat. No. 2,692,980, entitled ENGINE CONDITION SIGNAL ARRANGEMENT FORAUTOMOTIVE VEHICLES and issued on Oct. 26, 1954, to Platt; U.S. Pat. No.2,825,418, entitled MOTOR VEHICLE ACCELERATION SIGNAL DEVICE and issuedon Mar. 4, 1958, to Kershman; and U.S. Pat. No. 3,958,542, entitledTHROTTLE CONTROL APPARATUS USING PEDAL RESISTANCE and issued on Mar. 25,1976, to Tanner.

With compression ignition or diesel internal combustion engines whichare fuel injected, fuel flow rates are controlled by throttle position.Efficiency of operation and hence maximization of fuel economy isdirectly proportioned to throttle position. Intake manifold vacuumlevels in a diesel are essentially nonexistent. As a result, signalingdevices of the aforementioned type would not properly indicateinefficient vehicle operation. Automatic, constant speed cruise controlsystems which have been used with diesel engines do not optimize fuelefficiency under increasing load conditions. Typical diesel cruisecontrol systems are of the same type as have been used on spark ignitionengines and which are described above. Such engines include, however, amechanically driven vacuum pump as a vacuum source.

A need exists for a device and system which will allow a variablevehicle speed within preset limits under light and moderate loadconditions but will seek to maintain intake manifold vacuum levels at orabove a preset point or throttle positions at or below a preset point toincrease efficiency of engine operation when increased and/or high loadconditions are experienced. Such a system should also soften or moderatethe rapid and wasteful throttle movements which have heretofore beenexperienced with automatic cruise control systems.

SUMMARY OF THE INVENTION

In accordance with the present invention, the aforementioned need isfulfilled. Essentially, a modulator means is provided which is adaptedto be connected in series with the source of vacuum and the servotransducer of a vehicle cruise control system. The modulator meanslimits opening movement of the throttle causing variable vehicle speedand permitting efficient operation to be maintained. In narrower aspectsof the invention, application of vacuum to the actuator is preventedwhen a predetermined minimum intake manifold vacuum level or maximumthrottle position is reached. The modulator means and system inaccordance with the present invention "softens" the heretoforeexperienced rapid throttle movements characteristic of automatic cruisecontrol systems. Since intake manifold levels and/or throttle positionare maintained in an efficient range, increased fuel economy isachieved.

In further aspects of the invention, as presently preferred, themodulator means is a vacuum actuated valve having an inlet connected tothe cruise control system source of vacuum and an outlet connected tothe servo transducer which controls the passage of vacuum to the vacuumservo or throttle positioner. A valve element is positioned in responseto throttle position and/or intake manifold vacuum. When a predeterminedset point is reached, the valve element is closed, thereby blockingcommunication of the source of vacuum to the servo actuator. The setpoint of operation of the modulator means is adjustable by the vehicleoperator. The modulator means is readily incorporable into commerciallyavailable automatic cruise control systems and permits such systems tomaximize fuel efficiency under varying load conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a cruise control system inaccordance with the present invention incorporating a vacuum modulatorwhich is responsive to intake manifold vacuum levels;

FIG. 2 is an elevational view in cross section of the modulator meansincorporated in the system of FIG. 1;

FIG. 3 is a schematic illustration of a cruise control system inaccordance with the present invention incorporating a vacuum modulatorwhich is responsive to throttle position and which is thereforeprimarily adapted for use with diesel or compression ignition internalcombustion engines; and

FIG. 4 is an elevational view in cross section of the modulator meansincorporated in the system of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 schematically illustrates an automatic vehicle cruise controlsystem which is primarily adapted for use with a spark ignition,internal combustion engine. The system, generally designated 10,includes a vacuum reservoir or storage tank 12 connected to a vehicleintake manifold 14 through tubing 16. The source of vacuum 12 isconnected to a servo transducer or control unit 18 through tubing 20.Control unit 18 is connected to a vacuum actuated servo 22 throughtubing 24. Servo 22 typically includes a spring loaded diaphragm shiftedin response to application of vacuum through unit 18. Actuator 22 iscoupled through linkage 26 to throttle 28 of a carburetor 30 of theengine. The system 10 further includes a speed sensor 32. Sensor 32senses actual vehicle speed and generates an electrical signaltransmitted to an electronic logic circuit or command module (ECM)generally designated 34. The electronic command module compares actualvehicle speed to a "set" speed which has been inputted by the vehicleoperator and generates an error signal which is transmitted tocontroller 18. If the set speed is above actual vehicle speed, unit 18,which contains a valve (not shown), will open to permit vacuum to beapplied to the servo 22, thereby causing the throttle 28 to open. If theset speed is below the actual vehicle speed, unit 18 shuts off vacuumand/or bleeds servo 22 so that throttle valve 28 will move towards aclosed position. The precise circuitry and structure of controller 18,speed sensor 32 and the electronic command module 34 are well-known inthe art and will not be described in detail here. If necessary,reference may be had to the aforementioned U.S. Pat. Nos. 3,485,316 and4,170,274.

VACUUM MODULATOR

In accordance with the present invention, system 10 includes a modulatormeans generally designated 50. In the embodiment of FIG. 1, modulatormeans 50 is positioned in series with unit 18 through tubing 20. Themodulator means functions to limit and/or cut off communication ofvacuum from reservoir 12 to unit 18 and hence servo 22 when the intakemanifold vacuum level drops to a predetermined minimum set point.

As illustrated in FIGS. 1 and 2, modulator 50 includes a valve body 52which defines a vacuum inlet 54, a vacuum outlet 56 and a control portinlet 58. Tubing 20 is connected to the vacuum inlet 54 downstream ofreservoir 12. Vacuum is communicated to unit 18 through vacuum outlet56. Control port 58 is connected through suitable tubing 60 to theintake manifold vacuum 14 so that actual intake manifold vacuum levelsare transmitted to the modulator 50.

As seen in FIG. 2, valve body 52 defines a diaphragm cavity 64 withinwhich is disposed a flexible wall or diaphragm 66. Diaphragm 66 isclamped between a generally circular flange portion 68 and an end plate70. Diaphragm 66, therefore, defines a first chamber 72 and a secondchamber 74. Plate 70 is secured to the main portion of housing 72 bysuitable fasteners 76. Plate 70 has another plate 71 secured theretowhich defines inlet port 54 and outlet port 56. A valve element 80 iscarried by diaphragm 66 and is positioned coaxially therewith. Valveelement 80 is shiftable towards and away from a valve seat 82. When inthe position shown in FIG. 2, communication between inlet 54 and outlet56 is closed off.

Control port 58 communicates with chamber 72. A resilient meansillustrated in the form of a coil spring 86 has an end 88 engaging valveelement 80 and hence diaphragm 66. An opposite end 90 of the resilientmeans engages an abutment plate 92 of an adjustment means generallydesignated 94. Adjustment means 94 in the embodiment illustratedcomprises an elongated rod 96 threadably disposed in a bore 98 which iscoaxial with spring 86, valve element 80 and diaphragm 66. An adjustmentknob 99 is nonrotatably secured to an end 100 of rod 96 which extendsout of valve housing 52.

As should be apparent, spring 86 biases the diaphragm and hence thevalve element towards a first or closed position. By adjusting rod 94,the preload that spring 86 exerts on diaphragm 66 may be varied. As aresult, the initial force that the spring exerts can be increased ordecreased through rotation of rod 94. Intake manifold vacuumcommunicating with port 58 through tubing 60 tends to move diaphragm 66towards a second or open position against the bias or force of spring 86so that inlet 54 and outlet 56 are in communication with each other.

OPERATION

In operation, the conventional components of the cruise control systemfunction in a "normal" fashion. Under light or moderate load conditions,the command module 34 controls servo 22 through control means 18 toposition the throttle valve 28 to maintain an essentially constantvehicle speed. Under such conditions, the intake manifold vacuum levelstransmitted to modulator 50 through tubing 60 are high enough to keepvalve element 80 off its seat 82. When increasing load conditions areencountered, such as when the vehicle approaches an incline, throttlevalve 28 will be moved towards a more open position resulting in adecrease in the intake manifold vacuum level. This decreased level istransmitted to modulator 50 causing the valve element to start to movetowards its first or closed position. When the predetermined set pointis reached, as adjustably set by adjustment means 94, the vacuum leveldrops below that necessary to hold valve element 80 away from its valveseat. Communication between inlet 54 and outlet 56 is prevented when thevalve element closes. As a result, the automatic cruise control system,while "calling" for increased throttle opening through the speed sensorand command module 84, is overridden and further opening of the throttlevalve by the servo is prevented. The vehicle speed will decrease, andintake manifold vacuum levels are maintained at or above the set point.This variable speed operation in response to load conditions increasesfuel efficiency.

Typically, the set point would be adjusted so that the valve 58 is openat vacuum levels equal to or above 10 to 13 inches Hg. Valve element 80will move from its second to its first position in direct response to adecrease in intake manifold vacuum levels and rapid or violent throttleposition changes are softened or modulated through a gradual decrease incommunication of vacuum to the controller 18. If the vehicle's speedshould drop below an acceptable level, such as when an unusually steepincline is encountered, the operator need merely depress the acceleratorpedal in a conventional fashion to override the system, open thethrottle and increase the vehicle's speed.

It is presently envisioned that the cruise control system couldincorporate a bypass line and valve around modulator 50. The valve wouldbe "normally closed" and electrically connected to additional logiccircuits into which a predetermined minimum vehicle speed would be set.When minimum actual speed is sensed by sensor 32, the bypass valve couldopen causing "normal" operation of the system. Also, the logic circuitscould directly control unit 18 and be coupled to module 34 so that uponreaching the minimum set speed, module 34 would be disabled and unit 18would be controlled by the additional logic circuits. The actuator 22would then be operated to maintain the minimum set speed. When vacuumlevels increase and/or load decreases, operation could be returned tomodule 34 and modulator 50.

DIESEL ENGINE MODULATOR

System 10, illustrated in FIGS. 1 and 2, is primarily adapted for usewith spark ignition, conventionally carbureted, internal combustionengines wherein intake manifold vacuum level is directly proportional toengine operating efficiency and fuel flow. In a diesel engine which isfuel injected, throttle position is the best indicator of fuelefficiency since throttle position is directly proportional to fuel flowand the brake mean effective pressure of the engine, except under"overfueling" conditions. Overfueling conditions are evidenced byincreased exhaust smoke, and increased fuel levels produce reduced poweroutput. In a diesel engine, there is no real or usable manifold vacuum,and throttle position is an accurate indicator of efficiency ofoperation.

The embodiment illustrated in FIGS. 3 and 4 is a modulated vehiclecruise control system primarily adapted for diesel or compressionignition internal combustion engines. The system as schematicallyillustrated in FIG. 3 is generally designated 110. As with system 10,the embodiment of FIG. 3 includes a vacuum source 111, a servotransducer or control unit 18, a vehicle speed sensor 32 and a commandmodule 34. Vacuum source 111 is a conventional, mechanically drivenvacuum pump and reservoir included to power the system and/or othervehicle accessories. The diesel engine also includes an air inlet 113having a throttle 115.

The modulator means is generally designated 150 in FIGS. 3 and 4.Modulator 150 replaces the vacuum actuated servo 22 of system 10.Modulator 150 operates in response to throttle position and includes avalve body 152, a vacuum inlet port 154, a vacuum outlet port 156 and adiaphragm control port 158. Inlet 154 is connected to the vacuumreservoir 12 through suitable tubing 160. Vacuum outlet 156 is connectedto control unit 18 through suitable tubing 162. Diaphragm control port158 is connected to control unit 18 through suitable tubing 164.Therefore, modulator 150 is placed in series with the cruise controlservo transducer or unit 18 and the source of vacuum 12.

As best seen in FIG. 4, valve body 152 of modulator 150 defines a firstcavity or diaphragm chamber 170 within which is supported a flexiblewall or diaphragm element 172. Diaphragm 172 defines a chamber 174 whichis placed in communication with vacuum through control port 158. Securedcentrally of diaphragm 172 is a bracket 176. Bracket 176 is welded orotherwise suitably secured to a pair of plates 178, 180 which aresandwiched about diaphragm 172. Bracket 176 is connected to the existingthrottle linkage via member 182 (FIG. 3). As should be apparent,application of vacuum to control port 158 moves diaphragm 172 to theleft, when viewed in FIG. 4, thereby opening the throttle 28.

Valve body 152 further defines an elongated bore 186 which is coaxialwith diaphragm 172. Bore 186 is stepped in cross-sectional configurationand defines an annular valve seat 188. Inlet 154 communicates through apassage 190 with bore 186 at valve seat 188. Similarly, vacuum outlet156 communicates with bore 186 at valve seat 188 through a passage 200.

Slidably disposed within bore 186 is an elongated, generally cylindricalvalve element generally designated 202. Valve element 202 includes agenerally circular portion 204 defining a valve face 206 carrying asuitable valve seat seal 208. Portion 204 also defines a seal groove 207within which is disposed an O-ring seal 209. Joined to circular portion204 is an elongated portion 212. Portion 212 defines a blind bore 214having a closed end 216 and an open end 218 which faces diaphragm 172.

Extending into blind bore 212 from diaphragm 172 is an elongated rod222. Positioned around rod 222 is a first resilient means 224,illustrated as a coil spring. Coil spring 224 includes a first end 226which abuts diaphragm 172 and a second end 228 which abuts the closedend 216 of bore 214. As should be readily apparent, spring 224 biases orgenerates a force tending to move valve element 202 towards a first orclosed position at which communication between inlet 154 and outlet 156through bore 186 is prevented.

Bore 186 opens through an outer face 236 of valve body 152. An innerperipheral surface 237 of the bore is threaded and adjustably receivesan externally threaded nut or member 238. Member 238 includes an innerface 240. A second resilient means 242 is positioned between face 240and a shoulder 244 defined by the valve element. Means 242 is shown as acoil spring and has a spring rate greater than that of spring 224.Spring 242 tends to bias or create a force which shifts the valveelement face 202 away from valve seat 188. The preload or the initialforce spring 242 exerts on the valve element is adjusted by movement ofadjustment member or nut 238 within the threaded portion of the bore186.

OPERATION

The operation of embodiment 110 is similar to that of embodiment 10 inthat modulator 150 cuts off communication of vacuum from source 111 tothe throttle positioning servo. Modulator 152 incorporates the throttlepositioning servo so that valve element closure occurs and responds toopening movement of the servo actuator.

Under light or moderate load conditions, vacuum from source 111 iscommunicated to chamber 174 by passing through inlet 154, outlet 156,control unit 18 and to diaphragm control port 158. The cruise controlsystem, therefore, operates in a "normal" fashion with throttle positionbeing set in direct response to a comparison of set speed with actualspeed. As increased load conditions are encountered and diaphragm 172moves further to the left, when viewed in FIG. 4, to further open thethrottle, an increased force is exerted on the valve element by thefirst resilient means 224. When the force created by spring 224overcomes the countering force created by spring 242, the valve willseat and close off the inlet port 154. As a result, vacuum from source11 will no longer communicate with the servo chamber 174 through controlunit 18 and further opening of the throttle valve 28 is prevented.

As with modulator 50, movement of the valve from a second, fully openposition towards a first or closed position modulates or "softens" harshthrottle movements which have heretofore been experienced with cruisecontrol systems when encountering a rapidly increasing load condition.The valve element in effect defines a variable restriction. The maximum,open throttle position is limited to a preset point. Inefficient engineoperation is reduced or eliminated. The specific set point at which thevalve element closes is readily adjustable by rotation of member 238.This increases the initial force or preload that spring 244 exerts onthe valve element. When the preload or initial force is increased,increased throttle movement in an opening direction is required beforefirst resilient means 224 generates enough force to overcome thatcreated by spring 244.

In view of the foregoing description, it should be readily apparent tothose of ordinary skill in the art that the vacuum and throttlemodulator in accordance with the present invention automatically limitthrottle opening to prevent inefficient engine operation. The modulatorsare readily incorporable into existing cruise control systems withoutmodification of such systems. The modulators are merely placed in linedownstream of the source of vacuum and upstream of the cruise controlservo transducer or control circuit which normally controls transmissionof vacuum to the vacuum servo actuator. In the embodiments of FIGS. 3and 4, the vacuum servo actuator of the conventional systems is removedand modulator 150 is substituted therefor. The systems obtain constantspeed operation at light or moderate load levels and automaticallyprevent inefficient operation when an increased load is encountered,such as when the vehicle encounters an incline.

Those of ordinary skill in the art will now undoubtedly envision variousmodifications to the present invention which would not depart from thepatentable concepts disclosed herein. For example, the specificconfiguration of the valve elements and the valve seats could bemodified without changing the functioning of the modulator. For example,the valve element could be a tapered needle which would close offcommunication of an inlet passage with an outlet passage. Other"flexible" walls means, such as a bellows, could be substituted for thediaphragm. Further, resilient means other than the coil springsillustrated could be substituted to create the required biasing forces.Therefore, it is expressly intended that the above description should beconsidered as that of the preferred embodiments only. The true spiritand scope of the present invention may be determined by reference to theappended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. In a vehicle cruisecontrol system of the type including a vacuum actuated servo connectedto an engine throttle valve, a source of vacuum and a control meansconnecting the source of vacuum to the servo for controlling throttleposition to maintain a preset vehicle speed, the improvement comprisingadjustable modulator means operatively connected to said control meansand said servo for automatically limiting opening movement of thethrottle by modulating communication of vacuum through said controlmeans to said servo to override said control means during normaloperation of said control means so that vehicle speed will drop belowthe preset vehicle speed which the control means seeks to maintain uponthe vehicle encountering an incline to increase fuel efficiency andsoften throttle position changes.
 2. In a vehicle cruise control systemas defined by claim 1 wherein said modulator means comprises:a valvebody defining an inlet port connected to the source of vacuum, an outletport connected to said control means and a valve element chamber; avalve element shiftable within said chamber between a first positionwherein said inlet port is closed and a second position wherein saidinlet port is open so that vacuum is communicated to said control means;and means engaging said valve element for shifting said valve element tosaid first position in response to opening movement of the throttlevalve to prevent further application of vacuum to said vacuum servothrough said control means when a predetermined throttle valve positionis reached, said throttle valve opening position being controlled bysaid servo.
 3. In a vehicle cruise control system as defined by claim 2wherein said vacuum actuated servo includes a housing, a diaphragmdisposed within said housing, a vacuum port connected to the source ofvacuum through said control means for communicating vacuum to one sideof the diaphragm, and a bracket secured to said diaphragm and connectedto the throttle.
 4. In a vehicle cruise control system as defined byclaim 3 wherein said means engaging said valve element comprises:a firstresilient means positioned between said servo diaphragm and said valveelement for biasing said valve element towards said first position asvacuum is communicated to said one side of said diaphragm; and secondresilient means disposed within said valve body and engaging said valveelement for biasing said valve element towards its second positionagainst the force generated by said first resilient means whereby whenthe servo diaphragm reaches a predetermined position, said firstresilient means will overcome the force created by said second resilientmeans and said valve element will move to its first position preventingcommunication of said source of vacuum to said servo diaphragm throughsaid control means.
 5. In a vehicle cruise control system as defined byclaim 4 wherein said modulator means further includes:adjustment meansengaging said second resilient means for adjusting the preload saidsecond resilient means exerts on said valve element.
 6. In a vehiclecruise control system as defined by claim 5 wherein said valve body andsaid servo housing comprise a single unit and said valve element ispositioned coaxially with said diaphragm.
 7. In a vehicle cruise controlsystem as defined by claim 6 wherein said first resilient meanscomprises a coil spring having a first spring rate, said coil springhaving an end engaging said diaphragm and another end engaging saidvalve element.
 8. In a vehicle cruise control system as defined by claim7 wherein said second resilient means comprises a second coil springhaving a second spring rate, said second coil spring having an endengaging said valve element and another end engaging said adjustmentmeans.
 9. In a vehicle cruise control system as defined by claim 7wherein said adjustment means comprises said valve body defining athreaded bore coaxial with said second coil spring and an externallythreaded member threadably disposed within said bore and having a faceengaging said another end of said second coil spring.
 10. A throttleposition modulator for use in a vehicle cruise control system of thetype including a source of vacuum, said modulator comprising:a valvebody defining a diaphragm chamber, a control port opening into saidchamber and adapted to be connected to a control valve, said bodyfurther defining a vacuum inlet port, a vacuum outlet port and a valveseat, said vacuum inlet and outlet ports opening through said valveseat; a diaphragm supported within said diaphragm chamber; throttleconnecting means secured to said diaphragm for connecting said diaphragmto a throttle; a valve element movably supported within said valve bodyand movable between a first position engaging the valve seat and closingsaid vacuum inlet port and a second position away from said valve seatso that said vacuum inlet port communicates with said vacuum outletport; and set point means engaging said valve element for causing saidvalve element to move to its first position in response to movement ofsaid diaphragm.
 11. A throttle position modulator as defined by claim 10wherein said set point means comprises:first resilient means engagingsaid diaphragm and said valve element for resiliently biasing said valveelement towards its first position and creating a force tending to movesaid valve element to its first position, which force increases withmovement of the diaphragm; and second resilient means supported withinsaid body and engaging said valve element for resilient biasing saidvalve element towards its second position against the bias of said firstresilient means.
 12. A throttle position modulator as defined by claim11 further including adjustment means engaging said second resilientmeans for adjusting the bias of said second resilient means on saidvalve element whereby the set point at which said valve element moves toits first position in response to movement of said diaphragm may beadjusted.
 13. A throttle position modulator as defined by claim 12wherein said valve body defines an elongated bore which is coaxial withsaid diaphragm, said valve element being generally circular in crosssection and being slidably disposed within said bore, said valve bodyfurther defining an annular shoulder defining said valve seat.
 14. Athrottle position modulator as defined by claim 13 wherein said firstresilient means comprises a coil spring having an end engaging saiddiaphragm and another end engaging said valve element.
 15. A throttleposition modulator as defined by claim 14 wherein said second resilientmeans comprises another coil spring having an end engaging said valveelement in opposed relationship to said first resilient means.
 16. Athrottle position modulator as defined by claim 15 wherein saidadjustment means comprises a threaded member threadably disposed withinsaid bore for movement towards and away from said valve element, saidanother coil spring having an end engaging said threaded member.
 17. Ina vehicle cruise control system of the type including a throttlepositioner connected to an engine throttle for opening the throttle andcontrol means sensing actual vehicle speed and actuating the throttlepositioner to maintain vehicle speed at a preset speed, the improvementcomprising:adjustable modulator means operatively connected to saidcontrol means for overriding said control means at an adjustable setpoint during normal operation of said control means to soften throttlemovement and limit opening movement of the engine throttle causing adecrease in vehicle speed below the preset speed to increase fuelefficiency when the vehicle encounters an incline.
 18. In a vehiclecruise control system as defined by claim 17 wherein the system furtherincludes a source of vacuum and the throttle positioner is connected tothe source of vacuum through said control means and said modulator meansincludes a valve means interposed between said source of vacuum and saidthrottle positioner for shutting off communication between said sourceof vacuum and said throttle positioner.